vii OX THE INFERTILITY OF CROSSES 167 



law existed was never more than a plausible generalisa- 

 tion, founded on a few inconclusive facts derived from 

 domesticated animals and cultivated plants. The facts were, 

 and still are, inconclusive for several reasons. They are 

 founded, primarily, on what occurs among animals in 

 domestication; and it has been shown that domestication 

 both tends to increase fertility, and was itself rendered 

 possible by the fertility of those particular species being little 

 affected by changed conditions. The exceptional fertility of 

 all the varieties of domesticated animals does not prove that 

 a similar fertility exists among Datura] varieties. In the next 

 place, the generalisation is founded on too remote crosses, as in 

 the case of the horse and the ass, the two most distinct and 

 widely separated species of the genus Equus, so distinct indeed 

 that they have been held by some naturalists to form distinct 

 genera. Crosses between the two species of zebra, or even 

 between the zebra and the quagga, or the quagga and the ass, 

 might have led to a very different result A.gain, in pre- 

 Darwinian times it was so universally the practice to argue in 

 a circle, and declare that the fertility of the offspring of a 

 cross proved the identity of species of the parents, that experi- 

 ments in hybridity were usually made between very remote 

 species and even between species of different genera, to avoid 

 the possibility of the reply : " They are both really the same 

 species ;" and the sterility of the hybrid offspring of such 

 remote crosses of course served to strengthen the popular 

 belief. 



Now that we have arrived at a different standpoint, and 

 look upon a species, not as a distinct entity due to special 

 creation, but as an assemblage of individuals which have become 

 somewhat modified in structure, form, and constitution so as 

 to adapt them to slightly different conditions of life ; which 

 can be differentiated from other allied assemblages ; which 

 reproduce their like, and which usually breed together — we 

 require a fresh set of experiments calculated to determine the 

 matter of fact, — whether such species crossed with their near 

 allies do always j^roduce offspring which are more or less 

 sterile inter se. Ample materials for such experiments exist, 

 in the numerous "representative species " inhabiting distinct 

 areas on a continent or different islands of a group : or even 



